Organic LEDs brighten up

04 Jul 2007

Magnetic nanoparticles could boost the efficiency of an organic light-emitting device by more than 30%, say US researchers.

Jian Shen of the Oak Ridge National Lab and colleagues used the magnetic nanoparticles to dope the structure of a polymer-based organic light-emitting diode (OLED). The technique not only opens up a way to get more light out of an OLED, but also allows the OLED intensity to be controlled by an external magnetic field.

CoFe nanoparticles on substrates

A typical polymer-based OLED structure contains three layers: a thin light-emitting layer held between a hole-transport layer and an electron-transport layer. The emissive layer should be thin enough to allow the electrons and holes from the transport layers to meet and recombine.

Shen and colleagues fabricated their device by using an ultrasound method to mix cobalt ferrite (CoFe) nanodots into chloroform solutions of polymers. The researchers spin-cast the CoFe-doped polymers onto a conducting glass substrate to form the OLED. They then measured the electroluminescence intensity of the doped OLED and compared it with that of a non-doped OLED.

The team found that the quantum efficiency of the OLED increased by 27% for an OLED that was doped with 0.1% of nanoparticles – a figure that rose to 32% when an external magnetic field was applied. According to the researchers, these improvements can be attributed to two simultaneous effects: an increase in the number of excitons among the total number of charge carriers, and an increase in the fraction of “singlets” among the total number of excitons. Singlets are electron-hole pairs with opposite spins, so that the total spin equals zero.

“The high efficiency of OLEDs enhanced by doping with CoFe nanoparticles could play an important role in accelerating the commercialization of OLEDs for other applications, such as magnetic-field sensors,” explained Shen. “Moreover, the magnetic tuneability implies that the new OLED can be controlled via a non-contact method (an external magnetic field).”

The researchers are now trying to optimize their process to further enhance the quantum efficiency of the OLED. They will do this by adjusting the doping concentration and also by making more uniform magnetic nanoparticles.

The work was published in Applied Physics Letters.